The cowpea (Vigna unguiculata) is an annualherbaceouslegume from the genus Vigna. Due to its tolerance for sandy soil and low rainfall it is an important crop in the semi-arid regions across Africa and other countries. It requires very few inputs, as the plant's root nodules are able to fix atmospheric nitrogen, making it a valuable crop for resource-poor farmers and well-suited to intercropping with other crops. The whole plant is used as forage for animals, with its use as cattle feed likely responsible for its name.

Four subspecies of cowpea are recognised, of which three are cultivated. There is a high level of morphological diversity found within the species with large variations in the size, shape and structure of the plant. Cowpeas can be erect, semi-erect (trailing) or climbing. The crop is mainly grown for its seeds, which are extremely high in protein, although the leaves and immature seed pods can also be consumed.

Cultivated cowpeas are known by the common names black-eyed pea, southern pea, yardlong bean, catjang, and crowder pea. They were domesticated in Africa and are one of the oldest crops to be farmed. A second domestication event probably occurred in Asia, before they spread into Europe and the Americas. The seeds are usually cooked and made into stews and curries, or ground into flour or paste.

Most cowpeas are grown on the African continent, particularly in Nigeria and Niger which account for 66% of world production. A 1997 estimate suggests that cowpeas are cultivated on 12.5 million hectares (31 million acres), have a worldwide production of 3 million tonnes and are consumed by 200 million people on a daily basis.[4] Insect infestation is a major constraint to the production of cowpea, sometimes causing losses of over 90% loss in yield.[5] The legume pod borer Maruca vitrata is the main pre-harvest pest of the cowpea and the cowpea weevil Callosobruchusmaculatus the main post-harvest pest.

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Black-eyed peas, a common name for a cowpea cultivar, are named due to the presence of a distinctive black spot on their hilum.

Vigna unguiculata is a member of the Vigna (peas and beans) genus. Unguiculata is Latin for "with a small claw", which reflects the small stalks on the flower petals.[6] All cultivated cowpeas are found within the universally accepted V. unguiculata subspecies unguiculata classification, which is then commonly divided into four cultivar groups: Unguiculata, Biflora, Sesquipedalis, and Textilis.[7][8] Some well-known common names for cultivated cowpeas include black-eye pea, southern pea, yardlong bean, catjang, and crowder pea.[9] The classification of the wild relatives within V. unguiculata is more complicated, with over 20 different names having been used and between 3 and 10 subgroups described.[7][10] The original subgroups of stenophylla, dekindtiana andtenuis appear to be common in all taxonomic treatments, while the variations pubescens and protractor were raised to sub-species level by a 1993 charactisation.[7][11]

The first written reference of the word 'cowpea' appeared in 1798 in the United States.[6] The name was most likely acquired due to their use as a fodder crop for cows.[9] Black-eyed pea, a common name used for the unguiculata cultivar group, describes the presence of a distinctive black spot at the hilum of the seed. Black-eyed peas were first introduced to the southern states in the United States and some early varieties had peas squashed closely together in their pods, leading to the other common names of southern pea and crowder-pea.[6] Sesquipedalis in Latin means "foot and a half long", and this subspecies which arrived in the United States via Asia is characterised by unusually long pods, leading to the common names of yardlong bean, asparagus bean and Chinese long-bean.[12]

There is a large morphological diversity found within the crop, and the growth conditions and grower preferences for each variety vary from region to region.[7] However, as the plant is primarily self pollinating, its genetic diversity within varieties is relatively low.[13] Cowpeas can either be short and bushy (as short as 20 centimetres (7.9 in)) or act like a vine by climbing supports or trailing along the ground (to a height of 2 metres (6.6 ft)).[14][15] The tap root can penetrate to a depth of 2.4 metres (7.9 ft) after eight weeks.[16]

The size and shape of the leaves vary greatly, making this an important feature for classifying and distinguishing cowpea varieties.[17] Another distinguishing feature of cowpeas is the long 20–50 centimetres (7.9–19.7 in) peduncles which hold the flowers and seed pods. One peduncle can support four or more seed pods.[16] Flower colour varies through different shades of purple, pink, yellow and white and blue.[15]

Seeds and seed pods from wild cowpeas are very small,[16] while cultivated varieties can have pods between 10 and 110 centimetres (3.9 and 43.3 in) long.[18] A pod can contain 6–13 seeds that are usually kidney shaped, although the seeds become more spherical the more restricted they are within the pod.[14][16] Their texture and colour are very diverse. They can have a smooth or rough coat and be speckled, mottled or blotchy. Colours include white, cream, green, red, brown and black or various combinations.[16]

Compared to most other important crops, little is known about the domestication, dispersal and cultivation history of the cowpea.[19] Although there is no archaeological evidence for early cowpea cultivation, the centre of diversity of the cultivated cowpea is West Africa, leading an early consensus that this is the likely centre of origin and place of early domestication.[20] New research using molecular markers has suggested that domestication may have instead occurred in East Africa and currently both theories carry equal weight.[19]

While it may be uncertain when cultivation began it is still considered one of the oldest domesticated crops.[21] Remains of charred cowpeas from rock shelters in Central Ghana have been dated to the second millennium BCE.[22] In 2300 BC the cowpea is believed to have made its way into South East Asia where secondary domestication events may have occurred.[8] From there they traveled north to the Mediterranean, where they were used by the Greeks and Romans.[23] The first written references to the cowpea were in 300 BC and they probably reached Central and North America during the slave trade through the 17th to early 19th centuries.[8][20]

Cowpeas thrive in poor dry conditions, growing well in soils up to 85% sand.[24] This makes them a particularly important crop in arid, semi-desert regions where not many other crops will grow. As well as an important source of food for humans in poor arid regions the crop can also be used as feed for livestock.[25] Its nitrogen fixing ability means that as well as functioning as a sole-crop, the cowpea can be effectively intercropped with sorghum, millet, maize, cassava or cotton.[26]

Seeds from the wild type cowpea are much smaller than the cultivated varieties

The optimum temperature for cowpea growth is 30 °C (86 °F), making it only available as a summer crop for most of the world. It grows best in regions with an annual rainfall of between 400–700 millimetres (16–28 in). The ideal soils are sandy and it has better tolerance for infertile and acid soil than most other crops. Generally 133,000 seeds are planted per hectare (54,000/acre) for the erect varieties and 60,000 per hectare (24,000/acre) for the climbing and trailing varieties. The grain can be harvested after about 100 days or the whole plant used as forage after approximately 120 days. Leaves can be picked from 4 weeks after planting.[27]

These characteristics, along with its low fertilisation requirements, make the cowpea an ideal crop for resource poor farmers living in the Sahel region of West Africa. Early maturing varieties of the crop can thrive in the semi-arid climate, where rainfall is often less than 500 millimetres (20 in). The timing of planting is crucial as the plant must mature during the seasonal rains.[28] The crop is mostly intercroped with pearl millet and plants are selected that provide both grain and fodder value instead of the more specialised varieties.[29]

Storage of the grains can be problematic in Africa due to potential infestation by post-harvest pests. Traditional methods of protecting stored grain include using the insecticidal properties of Neem extracts, mixing the grain with ash or sand, using vegetable oils, combining ash and oil into a soap solution or treating the cowpea pods with smoke or heat.[30] More modern methods include storage in airtight containers, using gamma irradiation, or heating or freezing the grain.[31] Temperatures of 60 °C (140 °F) kill the weevil larvae, leading to a recent push to develop cheap forms of solar heating that can be used to treat stored grain.[32] One of the more recent developments is the use a cheap reusable double bagging system (called PICs) that asphyxiates the cowpea weevils.[33]

Larva from the Maruca vitrata, commonly called the maruca pod borer, is one of the most damaging insect pests to the cowpea plant.

Insects are a major factor in the low yields of African cowpea crops, and they affect each tissue component and developmental stage of the plant. In bad infestations insect pressure is responsible for over 90% loss in yield.[5] The legume pod borer, Maruca vitrata, is the main pre-harvest pest of the cowpea.[34] Other important pests include pod sucking bugs, thrips and the post-harvest cowpea weevil, Callosobruchus maculatus.[5]

Severe C.maculatus, infestations can affect 100% of the stored peas and cause up to 60% grain loss within a few months.[37][38] The weevil generally enters the cowpea pod through holes pre-harvest and lays eggs on the dry seed.[39] The larvae burrow their way into the seed, feeding on the endosperm. The weevil develops into a sexually mature adult within the seed.[40] An individual bruchid will lay 20–40 eggs, and in optimal conditions each egg can develop into a reproductively active adult in 3 weeks.[41] The most common methods of protection involve the use of insecticides, the main pesticides used being carbomates, synthetic pyrethoids and organophosphates.[42]

Cowpea is susceptible to nematode, fungal, bacterial and virus diseases, which can result in substantial loss in yield.[43] Common diseases include blights, root rot, wilt, powdery mildew, root knot, rust and leaf spot.[44] The plant is susceptible to mosaic viruses, which cause a green mosaic pattern to appear in the leaves.[44] The cowpea mosaic virus (CPMV), discovered in 1959, has become a useful research tool.[45] CPMV is stable and easy to propagate to a high yield, making it useful in vector development and protein expression systems.[45] One of the plant's defenses against some insect attacks is the cow pea trypsin inhibitor (CpTI).[46]CpTI is the only gene obtained outside of B. thuringiensis that has been inserted into a commercially available genetically modified crop.[47]

Besides biotic stresses, cowpea also faces various challenges in different parts of the world like drought, heat and cold.[48] Drought lowers the growth rate and development and reduces yield ultimately, although cowpea is considered as moderately drought tolerant to most other crops.[49] Drought at the pre-flowering stage in cowpea can reduces the yield potential by 360 kg/ha.[50] Crop wild relatives are the prominent source of genetic material, which can be tapped to improve biotic/abiotic tolerance in crops.[51]International Institute of Tropical Agriculture (IITA), Nigeria and Institut de I’Environment ae de Recherches Agricoles (INERA) are working on cowpea crop wild relatives to tap the genetic diversity and transfer that into cultivars to make them more tolerant to different stresses and adaptive to climate change.[52]

Hoppin' John, also known as Carolina Peas and Rice, is a common meal in the rural Southern United States that is often served with fresh greens, and is especially popular on New Years Day.

Cowpeas are grown mostly for their edible beans, although the leaves, green peas and green pea pods can also be consumed, meaning the cowpea can be used as a food source before the dried peas are harvested.[53] Like other legumes, cowpeas are cooked to make them edible, usually by boiling.[54] Cowpeas can be prepared in stews, soups, purees and casseroles,[55][56] but the most common way to eat them is in curries.[55] They can also be processed into a paste or flour.[57] Chinese long beans can be eaten raw or cooked, but as they easily become waterlogged are usually sautéed, stir-fried, or deep-fried.[58]

A common snack in Africa is Koki or Moyin-Moyin, where the cowpeas are mashed into a paste and then wrapped in banana leaves.[59] They also use the cowpea paste as a supplement in infant formula when weaning babies off milk.[60] Slaves brought to America and the West Indies cooked cowpeas much the same way as they did in Africa, although many people in the American South considered cowpeas not suitable for human consumption.[61] A popular dish was Hoppin' John, which contained black-eyed peas cooked with rice and seasoned with pork. Over time cowpeas became more universally accepted and now Hoppin' John is seen as a traditional Southern dish ritually served on New Year's Day.[62]

Cowpeas seeds provide a rich source of proteins and calories, as well as minerals and vitamins.[57] This complements the mainly cereal diet in countries that grow cowpeas as a major food crop.[63] A seed can consist of 25% protein and has very low fat content.[64] Cowpea starch is digested more slowly than the starch from cereals, which is more beneficial to human health.[57] The grain is a rich source of folic acid, an important vitamin that helps prevent neural tube defects in unborn babies.[65]

The cowpea has often been referred to as "poor man's meat" due to the high levels of protein found in the seeds and leaves.[54] However, it does contain some anti-nutritional elements, notable phytic acid and protease inhibitors, which reduces the nutritional value of the crop.[57] Although little research has been conducted on the nutritional value of the leaves and immature pods, what is available suggests that the leaves have a similar nutrition value to black nightshade and sweet potato leaves, while the green pods have less anti-nutritional factors than the dried seeds.[57]

Most cowpeas are grown on the African continent, particularly in Nigeria and Niger which account for 66% of world cowpea production.[66] The Sahel region also contains other major producers such as Burkina Faso, Ghana, Senegal and Mali. Niger is the main exporter of cowpeas and Nigeria the main importer. Exact figures for cowpea production are hard to come up with as it is not a major export crop. It is rather difficult to estimate world cowpea production as it is always grown in mixture with other crops. But according to 1997 estimate, cowpeas are cultivated on 12.5 million hectares (31 million acres) and have a worldwide production of 3 million tonnes.[4] While they play a key role in subsistence farming and livestock fodder, the cowpea is also seen as a major cash crop by Central and West African farmers, with an estimated 200 million people consuming cowpea on a daily basis.[67]

According to the Food and Agriculture Organisation of the United Nations, as of 2012, the average cowpea yield in Western Africa was an estimated 483 kilograms per hectare (0.195 t/acre),[66] which is still 50% below the estimated potential production yield.[68] In some tradition cropping methods the yield can be as low as 100 kilograms per hectare (0.040 t/acre).[15]

Outside Africa, the major production areas are Asia, Central America, and South America. Brazil is the world's second-leading producer of cowpea seed, accounting for 17% of annual cowpea production, although most is consumed within the country.[69]